Since a typical metal-oxide semiconductor field-effect transistor (MOSFET) has a gate threshold voltage in the range of 1-6 volts, MOSFETs typically require a gate bias voltage in most design applications. For example, a XRF5015 or MRF5015 MOSFET manufactured by Motorola, Inc. will require a gate voltage of approximately 2.26 volts, producing a drain idle current (IDS) of approximately 100 MA in a typical design. Under these specific bias conditions, the MRF5015 amplifier will exhibit a gain of greater than 10 dB and a drain efficiency of greater than 55% when the MOSFET amplifier is operating in the range of 403 to 470 megahertz (MHz), when used in a radio frequency transmitter design. This very high gain MOSFET is potentially unstable at both very high frequency (VHF) and ultra-high frequency (UHF) designs. A typical approach to enhance the stability of the MOSFET in RF designs is to provide an input loading resistor network between the drain and gate of the MOSFET.
Device S-parameters can be used to calculate and predict the effects of these circuit techniques (input loading and feedback) for achieving stability. For example, measured at VDD=12.5V, quiescent drain current (IDQ)=100 milli-amperes (mA) a MRF5015 amplifier is stable into 5:1 VSWR, over line voltages from 11 volts to 16.6 volts, and operating in a frequency band from 403-470 MHz, at 25.degree. Celsius. However, at -30.degree. Celsius, the power amplifier breaks into oscillations, even into voltage standing wave ratio (VSWR) conditions of as low as 2:1. This instability becomes even worse as output power decreases to under one watt. These cold operating temperature, low power, stability problems are probably due to:
1. The changes in the gate threshold voltage, which is inversely related to temperature. These temperature changes have a larger effect on the IDQ of MOSFETs with high transconductance (gfs) such as the MRF5015 than MOSFETs having low gfs's. PA0 2. Variation in the input and output impedance's of the RF power amplifier with changes in output power levels. Theses variations are even larger with the wider bandwidth matching (403-470 MHZ) and high Q input impedance of devices such as the MRF5015.
As shown from the above discussion, a need exists for a variable power amplifier which can maintain stability under severe output mismatch conditions when operating over a wide operating bandwidth.